The present invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle, onto a suitable substrate) and related technologies. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to apparatus and methods (such as microlithography apparatus and methods) performed using an X-ray beam as an energy beam. Even more specifically, the invention pertains to X-ray sources that generate a beam of xe2x80x9csoftxe2x80x9d X-ray radiation for use in an X-ray microlithography apparatus, an X-ray microscope, an X-ray analysis device, or the like, and to microelectronic-device manufacturing methods utilizing such X-ray exposure technology.
X-ray sources that utilize a discharge plasma are small, produce a large X-ray flux, convert input electrical energy into X-rays at a higher efficiency than X-ray sources that utilize a laser-generated plasma, and are inexpensive. Hence, there has been substantial activity directed to the development of X-ray microscopes and X-ray microlithography apparatus that include a discharge-plasma X-ray source.
An exemplary discharge-plasma X-ray source is termed a xe2x80x9cDense Plasma Focusxe2x80x9d (DPF) source manufactured by Cymer, Inc., San Diego, Calif. Information concerning this source is available on the Internet home page of Cymer, Inc. (http://www.cymer.com/) in the paper by Partlo et al., xe2x80x9cEUV (13.5 nm) Light Generation Using a Dense Plasma Focus Device,xe2x80x9d presented at the SPIE 24th Annual International Symposium on Microlithography, 1999, incorporated herein by reference. See also, U.S. Pat. No. 5,763,930, incorporated herein by reference.
A schematic diagram of a DPF source is shown in FIG. 10. The DPF is enclosed in a vacuum chamber (not shown). An electric charge from a DC high-voltage power supply 700 is stored in a capacitor C0. After the capacitor C0 reaches full charge, the charge is transferred to a capacitor C1 by closing a switch 701 (comprising a thyristor, IGBT (Insulated Gate Bipolar Transistor), or the like). Charging of the capacitor C1 raises the voltage of the capacitor C1, and the voltage is applied between a concentrically arranged (coaxial) anode electrode 703 and cathode electrode 702. As the applied voltage nears a peak voltage, a hollow-cathode preionization source (not shown) initiates avalanche breakdown of molecules of a gas situated between the anode electrode 703 and cathode electrode 702. This causes an electrical discharge to begin between the electrodes 702, 703.
At the onset of discharge, the electrical current flowing from the capacitor C1 is momentarily held off by a saturable reactor SR until a uniform plasma sheath has formed at the base of the set of electrodes 702, 703. That is, at the surface of an insulator 704 disposed between the anode 703 and cathode 702, a uniform plasma sheath is generated by a surface discharge occurring at the insulator 704. As the capacitor C1 continues to dump electrical current across the electrodes 702, 703, the plasma sheath moves toward the tip of the anode electrode 703. The resulting magnetic forces generated at the tip of the anode electrode 703 compress the plasma in this region toward the axis. Also, as the plasma reaches the tip of the anode electrode 703, materials situated at the anode tip (e.g., gas molecules, the electrode material, or a target material) are energized sufficiently to enter the plasma. The plasma, compressed within the small region bounded by the strong magnetic field, undergoes further heating, causing soft X-ray (SXR) radiation to propagate therefrom. (SXR radiation also is termed herein xe2x80x9cextreme ultravioletxe2x80x9d or xe2x80x9cEUVxe2x80x9d radiation.)
The saturable reactor SR undergoes charge saturation as the plasma sheath moves between the electrodes 702, 703, or at least when the plasma sheath reaches the tip of the anode electrode 703. Saturation causes a large current to flow between the anode 703 and the cathode 702, resulting in further heating and compression of the plasma at the tip of the anode electrode 703.
In the X-ray source summarized above, the target material conventionally is a gas present in an atmosphere in the vicinity of the electrodes 702, 703. The target material also can be the material constituting the anode electrode, or a substance on the surface of the anode electrode.
With conventional technology, problems arise whenever a gas (present in the atmosphere in the vicinity of the electrodes) is used as the target material. Because SXR radiation usually is highly absorbed by matter, the pressure of the gas generally cannot be increased. At the same time, the pressure of the gas inside the vacuum chamber generally needs to be about 10 Torr or lower. (Whenever a gas is used as the target material, the gas pressure normally is kept low to reduce the density of the target material.) Unfortunately, with such a scheme, the intensity of the generated SXR radiation also is very weak. For example, the publications noted above describe experiments performed using 0.1 -Torr Xe and 0.2-Torr Ar as target-material gases. However, almost no difference is observed in the SXR spectra generated from these target materials because substantial SXR radiation also is generated by the electrode material (e.g., tungsten). Hence, a gas is not optimal for use as a target material.
Whenever a solid electrode is used to supply the target material, the electrode tends to melt and be eroded away by the large electrical current applied to the electrode from the discharge. This results in rapid substantial changes of electrode shape, rendering long-term stable operation impossible. A melted or eroded electrode requires frequent interruption in use to perform electrode replacement, resulting in decreased operating efficiency. In addition, as the electrode is melted or eroded, material released from the electrode tends to migrate to and deposit on neighboring optical components and other structures. Deposits of such materials tend to reduce the optical performance (e.g., reflectivity or transmittance) of the affected optical components.
If the target material is situated on the anode electrode 703, then a distinctive spectrum of SXR radiation (i.e., a spectrum distinctive of the target material) can be generated. However, the target material conventionally must be applied to the electrode (and periodically replenished) by a discrete operation including shut-down of the apparatus, which renders continuous long-term operation impossible. An exemplary target material, as discussed in the Partlo et al. reference cited above, is lithium (Li); another exemplary target material, as discussed in the JP ""195 patent document cited above is lithium hydroxide (LiH). Unfortunately, both substances are highly reactive and dangerous, and are difficult to handle. Also, both of these substances react violently with water. Whenever atmospheric air is introduced into the vacuum chamber containing these target materials, a substantial risk is created of residual Li or LiH in the vacuum chamber reacting explosively with water vapor in the air.
The shortcomings of conventional X-ray-generation devices as summarized above extend also to X-ray microlithographic apparatus that comprise such X-ray-generation devices as a source of SXR radiation used for pattern transfer. i.e., the apparatus frequently must be shut down temporarily for electrode servicing. As a result, microelectronic-device fabrication using such apparatus tends to exhibit disappointingly low throughput.
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide X-ray-generation devices that can generate SXR radiation in a manner characterized by long-term stability both when using an electrode as the target material and when using a substance other than the electrode as the target material. Another object is to provide X-ray-generation devices that cause less reduction in optical performance, of an X-ray optical system used in conjunction with the X-ray-generation device, when the electrode or components peripheral to the electrode are eroded by high current or plasma. Yet another object is to provide X-ray-generation devices that are less dangerous to handle than conventional devices. Yet another object is to provide X-ray microlithography apparatus including such devices and microelectronic-device fabrication methods that, by utilizing such apparatus, exhibit improved throughput compared to conventional apparatus.
To such ends, and according to a first aspect of the invention, X-ray-generation devices are provided. An embodiment of such a device comprises first and second electrodes configured and situated relative to each other such that a voltage pulse applied across the electrodes forms a first plasma between the electrodes and concentrates the first plasma at a location relative to the first electrode. The xe2x80x9clocationxe2x80x9d includes a target material that is energized by the concentrated first plasma sufficiently to be consumed and to cause a second (and concentrated) plasma of the target material to emit soft X-ray (SXR) radiation having a spectral profile characteristic of the target material. The device also includes means for supplying, during operation of the X-ray-generation device, the target material to the location as the target material is consumed by the concentrated plasma. The means for supplying the target material can be configured to provide the target material xe2x80x9ccontinuouslyxe2x80x9d or xe2x80x9cintermittentlyxe2x80x9d (defined below) as the target material is consumed.
Any of various target materials can be used, which can be in a gaseous, liquid, or in a solidified state. For example, if the target material is a gas, then the means for supplying the target material can comprise a gas vessel and a valve, wherein the valve is situated and configured to release gas controllably from the gas vessel to the location. More specifically, the first electrode can be hollow and having a first end connected to the valve, a second end opening at the location, and a lumen connecting the first and second ends. In such a configuration, the valve can be situated and configured to release gas controllably from the gas vessel through the lumen and the second end of the hollow electrode to the location. This configuration also can include a means for recovering unconsumed target-material gas for subsequent recycling to the means for supplying the target material.
If the target material is a liquid, then the means for supplying the target material can comprise a reservoir and a nozzle, wherein the nozzle is situated and configured to discharge the target material from the reservoir to the location in a controllable manner. More specifically, the nozzle can be configured to discharge the target material as a continuous liquid stream or as discrete droplets to the location. This configuration also can include means for recovering unconsumed target material for subsequent recycling to the means for supplying the target material. For example, the first electrode can have a hollow configuration having a first end opening at the location, a second end, and a lumen connecting the first and second ends. In such a configuration, the means for supplying the target material can be configured to direct the target material to the first end such that unconsumed target material enters the first end and passes through the lumen to the second end. The means for recovering unconsumed target material can comprise a trap situated and configured to collect unconsumed target material exiting the second end. The means for recovering unconsumed target material also can include a means for recycling collected unconsumed target material from the trap to the reservoir.
If the target material is a solidified material, then the means for supplying the target material can comprise a reservoir and a nozzle, wherein the nozzle is situated and configured to discharge particles of the target material controllably from the reservoir to the location. More specifically, the first electrode can have a hollow configuration having a first end opening at the location, a second end, and a lumen connecting the first and second ends. In such a configuration, the means for supplying the target material can be configured to direct the target material to the first end such that unconsumed target material enters the first end and passes through the lumen to the second end. The means for recovering unconsumed target material can comprise a trap situated and configured to collect unconsumed target material exiting the second end. The means for recovering unconsumed target material also can include a means for recycling collected unconsumed target material from the trap to the reservoir.
The first electrode in the X-ray-generation device can be made at least partially of the target material that is consumed in the concentrated plasma at the location, that otherwise subjects the first electrode to erosion. In such a configuration, the means for supplying the target material can comprise a mechanism situated and configured to replenish the first electrode during erosion of the first electrode. The mechanism also can include a linear actuator attached to the first electrode, wherein the linear actuator is configured to move the first electrode in a manner serving to compensate for erosion of the first electrode.
The first electrode can be made of the solid target material, wherein the first electrode has a proximal end located adjacent the location, and a distal end. In such a configuration, the mechanism can comprise an extruder having an output connected to the distal end. The extruder can be configured to extrude an additional length of the first electrode to compensate for erosion of the first electrode. The extruder can include a furnace that provides a source of liquid target material. In such a configuration, the extruder includes an extrusion die and a means for urging the liquid target material through the extrusion die. The extruded liquid solidifies at the distal end in a manner serving to increase a length of the first electrode sufficiently to restore the proximal end adjacent the location.
The X-ray-generation device also can include a first-electrode monitor situated and configured to detect a position of the first electrode during use. Hence, a need for replenishing the first electrode is detected. Such detection can be used to direct the mechanism to replenish the first electrode as required to compensate for erosion of the first electrode.
According to another embodiment, an X-ray-generation device according to the invention comprises first and second electrodes configured and situated relative to each other such that a voltage pulse applied across the electrodes forms a plasma between the electrodes. The plasma is concentrated at a location adjacent the first electrode (e.g., anode electrode). The device also includes a means for providing a target material at the location so as to cause the concentrated plasma to produce soft-X-ray radiation having a spectral profile characteristic of the target material. The device also includes a means for replenishing, during operation of the X-ray-generation device, the first electrode (e.g., anode electrode) as it is eroded during operation of the X-ray-generation device.
Replenishment of the first electrode can be either xe2x80x9ccontinuousxe2x80x9d or xe2x80x9cintermittent.xe2x80x9d Either way, if the electrode were to be eroded by the plasma, an equivalent amount of the electrode material would be replenished without having to interrupt operation of the device. As used herein, xe2x80x9ccontinuousxe2x80x9d replenishment means replenishment without interruption over a period of time, and xe2x80x9cintermittentxe2x80x9d replenishment means replenishment with interruptions over a period of time. Neither necessarily means continuous replenishment over an infinitely long period of time.
The apparatus can include a means for controllably maintaining the tip of the first electrode at a substantially constant position (relative to the location) as the tip is eroded during operation of the X-ray-generation device. In this embodiment, for example, the position of the tip of an electrode subject to erosion is detected by a detector. Data from the detector can be used to control an electrode-replenishment device to maintain the tip at a xe2x80x9csubstantially constantxe2x80x9d position. Such control of the tip position allows the plasma-generation position and plasma-generation parameters to held substantially constant, thereby providing an X-ray source exhibiting stable characteristics over a long period of time. As used in this context, xe2x80x9csubstantially constantxe2x80x9d means that some amount of deviation is allowed within respective ranges for the plasma-generation position and plasma-generation parameters. Whenever these parameters are within the specified respective ranges, no corrective action need be taken. Whenever these parameters exceed the respective ranges, corrective action is initiated to return to within range.
According to another embodiment, an X-ray-generation device according to the invention comprises first and second electrodes configured and situated relative to each other such that a voltage pulse applied across the electrodes forms a plasma between the electrodes. The plasma is concentrated at a location adjacent the first (e.g., anode) electrode. The device includes a means for providing a target material at the location so as to cause the concentrated plasma to produce soft-X-ray radiation having a spectral profile characteristic of the target material. The device also includes means for xe2x80x9ccontinuouslyxe2x80x9d or xe2x80x9cintermittentlyxe2x80x9d (see above) supplying the target material during operation of the X-ray-generation device.
Generally, in this embodiment, the target material is a liquid or solid. If the target material is supplied continuously, operation of the X-ray-generation device can be continuous over a long period of time. However, plasma generation typically is pulsatile, so it is not necessary to supply target material continuously; intermittent supply is sufficient. Hence, even an xe2x80x9cintermittentxe2x80x9d supply of target material allows continuous operation of the device over a long period of time. As described elsewhere herein, liquid or solid target materials can be suspended in a carrier gas. The carrier gas itself can be a target material.
The device also can include a trigger mechanism connected to the electrodes and configured to control a timing with which the voltage pulses are applied to the electrodes. Hence, a plasma is produced at the location or a plasma previously generated between the electrodes arrives at the location synchronously with provision of the target material at the location or with a moment in which a density of the target material at the location exceeds a predetermined threshold density. This embodiment is especially useful when the target material is supplied intermittently. Hence, the arrival of target material at the location or the target-material density at the location being at or above a specified density (in the case of a gas) is detected. Detection can be performed directly using a detector or indirectly. The timing of the high-voltage pulse applied between the electrodes is controlled so that plasma is generated at the proper time. Also, in cases in which the generated plasma moves and is focused at or near the xe2x80x9clocationxe2x80x9d, as in the DPF system, the timing can be controlled such that the plasma is present at the location at exactly the instant the target material reaches the location where the plasma sheath is highly confined (i.e., where the plasma is xe2x80x9cfocusedxe2x80x9d). Hence, the target material is converted into plasma under optimal conditions, thereby maximizing the intensity of the generated SXR radiation.
xe2x80x9cDirectxe2x80x9d detection of the arrival of target material, as noted above, is performed using a detector. xe2x80x9cIndirectxe2x80x9d detection is based on, e.g., an operational timing of valves intermittently controlling the supply of target material, or based on a detection that the density of target material at or near the location is at or above a specified density.
Each of various other embodiments of X-ray-generation devices according to the invention includes first and second electrodes as summarized above. One embodiment includes a target-material supply mechanism configured to introduce, during operation of the device, the target material as a supersonic gas stream at the location so as to cause the concentrated plasma to produce soft-X-ray radiation having a spectral profile characteristic of the target material. This embodiment is especially useful when the target material is a gas. Supplying such a target material as a supersonic gas stream provides adiabatic cooling of the target material. As a result, atoms of the target material are attracted to one another by van der Waals forces and thus become large cluster molecules. Formation of a plasma from cluster molecules increases the intensity of the generated X-rays. Also, cluster molecules discharged from a nozzle exhibit little divergence, thereby maintaining high density of target material at the location where the plasma sheath is focused, and maximizing the intensity of the generated X-rays.
In another embodiment, the target-material supply mechanism is configured to suspend particles of a target material in a carrier gas and to discharge the suspension at the location during operation of the device. This embodiment is especially useful when the target material is a solid or liquid. If the particle (or droplet) size is small, then the aggregate surface area is relatively large compared to when the particle (or droplet) size is large. Hence, small particle (or droplet) size allows a faster increase in temperature of the target material when introduced into the plasma, and thus produces a higher-temperature plasma. (Particle size need not be uniform.) Also, individual particles tend to form their own separate microplasmas, so drops in plasma temperature due to thermal conduction can be alleviated. These factors allow a high-temperature plasma (with accompanying high intensity of produced X-rays) to be maintained. Use of a carrier gas also allows units of target material that do not contribute to the plasma to be carried away in the gas stream. The carrier gas desirably has a high transmittance to the wavelength of the produced X-rays.
In another embodiment, the target-material supply mechanism is configured to deliver, during operation of the device, a focused stream of a target material at the location. The stream can be of discrete liquid droplets of target material, discrete particles of the target material, a continuous liquid stream of target material, or a continuous gaseous stream of target material. Here, xe2x80x9ca focused stream at the locationxe2x80x9d means directing the stream of target material to the location such that the stream does not spread at any other location, and directing the stream from a relatively wide nozzle so that the target material collects at the location near the electrode tip. Near the electrode tip is where the plasma is generated and, in a DPF device, where the plasma sheath is focused. If the target material is directed to and focused at such a location, then substantially all the target material arriving at the location with the plasma sheath contributes to plasma generation, yielding a high intensity of SXR radiation.
In another embodiment, the target-material supply mechanism is configured to deliver, during operation of the device, a target material from at least one of the electrodes to the location. Hence, the electrode itself is used as a source of target material. As noted above, a plasma is generated in the location near the electrode tip. Hence, in this embodiment, the target material can be conveyed efficiently from the electrode into the plasma. Such a scheme increases plasma-formation efficiency and thus the intensity of produced X-rays. The effect is enhanced especially if the electrode that generates the plasma at the location or the electrode that focuses the generated plasma at the location is used as the supply of target material. Alternatively, target material can be supplied from another solid source near the location where plasma is generated, wherein the alternative source is not the electrode that generates the plasma or the electrode that focuses the generated plasma at the location.
In yet another embodiment, the target-material supply mechanism is configured to deliver, during operation of the device, lithium hydroxide to the location, wherein the lithium hydroxide is formulated as discrete solid particles, a suspension, a solution, or a gas. X-rays having a wavelength of 13.5 nm are generated whenever Li atoms are made into a plasma. Hence, for SXR microlithography, this embodiment is desirable. LiOH is preferred over Li or LiH due to the high reactivity of Li and LiH with water vapor. (LiOH has a much lower reactivity in this regard.) Also, LiOH can be used as a solid target material, as a liquid target material (dissolved in a solvent), or as a gaseous target material.
In yet another embodiment, the target-material supply mechanism is configured to deliver, during operation of the device, a target material to the location so as to cause the concentrated plasma to produce soft-X-ray radiation having a spectral profile characteristic of the target material. In this configuration, at least one of the electrodes, the insulating member, or other component exposed to the plasma is made of a material that is transmissive to the soft X-ray radiation. As electrodes and insulating materials are eroded in a plasma, debris from the erosion adheres to the surfaces of nearby optical components, which reduces the optical performance of those components. This embodiment avoids this problem by employing materials (as electrodes and/or insulators) that have a high transmittance to the X-ray wavelengths. Hence, if debris from such materials adheres to the surface of an optical member, the reduction in optical performance of the member is suppressed. This allows continuous use of the optical element over a longer period of time without replacement or human intervention. If the electrode and/or insulator includes a substance having a low X-ray transmittance, it should be possible to exhaust such debris readily. If the X-ray wavelength is 13.5 nm, then Si should be used for the insulator due to the high transmittance of this material. However, it is difficult to use Si alone as an insulator between electrodes. SiO2 (a Si compound) can be used instead. SiO2 made into a plasma breaks down to Si and oxygen. Oxygen does not have a high transmittance for 13.5-nm X-rays, but oxygen is gaseous and is exhausted readily without adhering to optical members.
According to another aspect of the invention, soft-X-ray (SXR) sources are provided. An embodiment of such a source includes a dense plasma focus (DPF) device configured to produce a plasma. The source also includes a target-material-supply mechanism configured to supply (continuously or intermittently), during operation of the dense plasma focus device, a target material to the plasma to cause the plasma to produce soft-X-ray radiation having a spectral profile characteristic of the target material. The SXR source can comprise multiple electrodes, wherein the target-material-supply mechanism includes a mechanism configured to replenish, during operation of the source, an electrode as the electrode is eroded during operation.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.